1. Field of the Invention
The present invention relates to a gas sensor placed in an exhaust system of an internal combustion engine and used for combustion control.
2. Description of the Prior Art
Gas sensors installed in an exhaust system of an internal combustion engine such as an automobile engine and used for combustion control include an O2 sensor, an air fuel ratio sensor, an NOx sensor, an HC sensor and the like. Elements built in these gas sensors include a cup-shaped gas sensor element comprising a cup-shaped solid electrolyte, and a laminated gas sensor element constituted by laminating ceramic sheets, electrodes and the like owing to limitations such as early activation or detection principles (e.g., Japanese Patent Application Publication No. H9-127050).
The gas sensor element provided in the gas sensor must be exposed to an exhaust gas for gas density measurement, while the gas sensor element must be protected against characteristic deterioration caused by poison of the exhaust gas or against an element crack caused by water splash. For such reasons, a cover is provided for covering the gas sensor element. The cover has gas passage holes on its side surface or the like to lead the exhaust gas into a measured gas room formed inside the cover.
Incidentally, it has conventionally been known that the constitution of the cover is contrived so that the flow velocity of the exhaust gas may not have an influence on an output of the gas sensor when the exhaust gas is led into the cover from the gas passage hole (Japanese Utility Model Publication (after examination) No. H3-4930).
Furthermore, it has been known that the constitution is contrived so that it may be difficult for water spattering through an exhaust pipe in which the gas sensor is provided to get inside the cover from the gas passage hole (Japanese Patent No. 2641346, Japanese Patent Application Publication No. H9-222416).
Still further, when the element is the laminated one, in some cases, a difference arises in a positional relation between a gas flow direction and a normal direction of the gas sensor element at the time of setting the gas sensor. In this case, the cover contrived to reduce the difference of response speeds (directionality) caused by setting state is also known (Japanese Patent No. 2653831). However, the covers of conventional constitutions present a problem of slowed response speed if the directionality or water coming in is reduced.
Further yet, another problem is that the difference of the response speeds (setting angle dependency) is large when the positional relation between an axial direction of the gas sensor element and the gas flow direction is different. For example, in Japanese Patent No. 2653831 mentioned above, the gas passage holes are disposed closer to a tip end side than a gas leading part of the gas sensor, so that a gas component desired to be detected needs to reach the measured gas room by using only turbulence diffusion. Therefore, the response speed is slowed down compared to the case of the cover having the constitution that leads the gas in by using a steady flow.
Further yet, in Japanese Utility Model Publication (after examination) No. H3-4930 mentioned above, because the flow of the measured gas inside an inner cover is not uniform, the directionality occurs in the response speed, causing the difference of the response speeds to be 10 ms or more. This makes a large difference of the characteristics depending upon the setting directions of the gas sensor, which might make it impossible to expect accurate gas density measurements. In Japanese Patent No. 2641346 mentioned above, because the gas passage hole is not provided in a bottom surface part of the inner cover, the amount of gas flowing into the inner cover is small, and the response speed is therefore slowed down. Further, as the flow in the inner cover is not uniform, the directionality emerges. Further, if the area of the gas passage hole is enlarged to increase the response speed, the element crack due to the water splash is more likely to occur, and moreover, when the element is heated by a heater to keep a constant temperature, there is a problem of increased power consumption.
Incidentally, in German Patent No. 19628423A1, the side surface of an outer cover does not have the gas passage holes, and it is designed to lead the gas that will flow into the inner cover from the gas passage hole provided on the bottom surface of the outer cover. However, because this gas passage hole is vertical to the gas flow, it has resistance to the gas inflow and poses a problem of the slowed response speed. Further, if a tip end portion of the element is set inclining to a downstream side of the exhaust gas flow, it raises a problem of the slowed response speed, and if it is set inclining to an upstream side, it raises a problem of deteriorated water splash resistance.
In both of German Patent No. 19628423A1 and German Patent No. 4436580A1, because the inner cover projects from the outer cover, the inner cover is exposed directly to the gas flow, and therefore the inner cover is cooled down. Since a radiant heat quantity from the gas sensor element is proportionate to the fourth power of the temperature of the inner cover, the power consumption increases when the inner cover is cooled down.
In Japanese Patent Application Publication No. H9-222416, since the bottom surface of the inner cover and the bottom surface of the outer cover are kept away, the exhaust gas that has got in from the gas passage hole on the side surface of the outer cover flows out from the gas passage hole on the bottom surface of the outer cover. This reduces the amount of gas flowing into the inner cover, and the response speed is thus slowed down.
In Japanese Patent Application Publication No. 2000-171429, if the tip end portion of the element is set inclining to the downstream side or the upstream side toward the flow of the exhaust gas, it raises a problem that responsiveness is slowed down. In other words, the setting angle dependency is increased, giving trouble in setting operation of the gas sensor.